I'm about to want to create a second entirely different dialog box
whose contents are described using the same dialog.h API as the main
config box. So I'm starting by moving as much handler code as possible
out of GenericMainDlgProc and its callers, and into a set of reusable
subroutines.
In particular, this gets rid of the disgusting static variables that
stored all the config-box state. Now they're stored in a more sensible
struct, which lives in the new context-pointer field provided by the
reworked ShinyDialogBox.
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